President Bush has pointed to hydrogen technology as the ultimate solution to the nation's fuel supply problems, but one big question waits to be answered: Where will all the hydrogen come from?

Even if manufacturers can produce affordable hydrogen-powered vehicles that people will want to buy, energy experts say the nation's petroleum addiction -- a key source of carbon emissions contributing to global warming -- won't end until an environmentally sound hydrogen supply and distribution system is at hand.

"If we don't generate hydrogen in an environmentally responsible way, we'd be going five steps backward, rather than forward," said Scott Samuelsen, director of the National Fuel Cell Research Center at UC Irvine, where he is also an engineering professor.

Samuelsen calls for a comprehensive national effort, backed by the kind of resources and coordination that resulted in the first moon landing, to map out a clean and sustainable energy future. The administration's $1.2 billion, five-year hydrogen initiative is a useful step, he added, but not nearly enough.

"We need to invest a lot more," he said. "I think it's doable, but it will take a unified strategy."

Making cars that work well on hydrogen may be the critical step needed to spur development of any new energy infrastructure. The reason there's no supply system readily apparent now is not so much a problem of technology, but simple economics, said Julio Friedmann, who runs a research program at Lawrence Livermore National Laboratory involving carbon emissions.

"There's still no demand for hydrogen," he said. "Until you have hydrogen cars, there's no reason to have pipelines and production."

Even if a storage scheme is found, it's still not entirely clear where all the hydrogen would come from to run any significant part of the national transportation system.

It's not that hydrogen is so hard to find. It's the most abundant element in the universe, the H in H2O -- ordinary water -- and the "hydro" in "hydrocarbon" -- as in natural gas or other fossil fuels. Hydrogen today, in fact, is produced almost entirely from natural gas.

The trouble is that it takes energy to get hydrogen in pure, usable form. That's why it's really not considered a fuel at all, but rather an energy carrier -- a method of storing power generated any number of ways.

The question is which way, and at what cost.

"It's just like electricity -- it's as clean or expensive as what you make it from," said Gene Berry, an engineer at Lawrence Livermore who analyzes hydrogen technologies.

Energy companies, academic scientists and government researchers are beginning to investigate the possibilities. Early outlines of a California "hydrogen highway" of refueling stations call for the first ones to carry hydrogen made from at least 20 percent renewable energy sources.

But no large-scale supply and distribution network will emerge until there's enough demand -- and that's going to take a while.

Optimists insist the technology problems can be solved -- eventually -- but nobody believes it will be easy.

Hydrogen already is being used to power reconfigured Prius hybrids, 30 of which are being delivered now to local-government fleets in pollution-sensitive Southern California, in a five-year demonstration project designed to help spread the word about hydrogen's potential.

These early hybrids may help prime the market, but they clearly aren't the ideal vehicles for mass consumption. The Irvine supplier, Quantum Fuel Systems Technologies Worldwide Inc., said the cars go only 70 to 80 miles before needing a refill, and cost $60,000 to equip -- not counting the cost of the Prius.

Vehicles equipped with hydrogen fuel cells -- quickly rechargeable battery-like units -- could someday replace internal-combustion engines altogether. That's only if engineers can find a way to make the fuel cells efficient enough to drive long distances, and cheap enough to be affordable. Fuel cell cars now cost on the order of $1 million apiece.

Efficient hydrogen storage is one of the toughest technical problems blocking headway.

"You have to have cost-effective hydrogen storage technology to cram enough on board a vehicle to get adequate driving range," said Jason Mark, vehicles director in Berkeley for the Union of Concerned Scientists, an advocacy group.

Say what you want about high prices at the pump. Gallon for gallon, nothing on the horizon even approaches the power and portability of gasoline.

"Gasoline is an incredibly, astoundingly energy-dense material," said Charles Chamberlin, co-director of the Schatz Energy Research Center at Humboldt State University. "We are going to be hard-pressed to find something that matches that. We've been incredibly fortunate, or damned, by our easy access to petroleum."

The advantage of hydrogen is that it offers another avenue, besides electric batteries, to load energy onto a moving vehicle. Although electric-car battery technology has improved, results have been disappointing even after decades of trial and error.

Many experts argue that the hydrogen production problems can be surmounted, possibly with refined versions of existing technologies. Friedmann, for instance, argues that serious attention be devoted to cleaner coal technology, an idea that involves capturing and sequestering all the carbon emissions underground.

Others doubt if coal can ever be made truly clean, and argue that novel energy sources have to be developed. At the National Renewable Energy Laboratory in Golden, Colo., government scientists are working on possibilities such as using a type of bioengineered algae to convert light into usable energy, which might then be fed into a hydrogen storage network.

"I truly believe this is going to work, but it's going to take time," said Maria Ghirardi, a senior scientist at the energy laboratory working on that project.

Wind and solar power are the most obvious clean energy sources for a hydrogen economy, but they come with their own well-known disadvantages, most notably the intermittent nature of the sun and wind and relatively low efficiencies. Still, most experts agree that renewables will have to play a role, touting hydrogen as the most promising way to store the power and smooth out the supply cycles.

Ultimately, experts said, the answer should be some combination of windmills and solar installations, if not high-tech algae and scrubbed-up coal, backed up by more efficient forms of current technology.

Just what sort of combination will depend on how the various technologies develop, and thus which one deserves the largest investment of scarce resources. Advocates like to tout the fact that hydrogen can be made from ordinary water. But as Californians can attest, even water can be hard to come by.